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Optical Access Networks
Access Networks and Full Service Providers
Architectures and Classification of Access Networks
Alternative Access Technologies
Fibre Access Networks: FTTCab, FTTC/B, FTTB/H
PON Architectures: AF, TPON, WPON, WRPON
APON/BPON, EPON and GPON
Future Developments
Optical Access Networks
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Access Networks and Full Service Providers
The networks feeding the metro (and core) networks by gathering data
from the end users (us).
The two main contenders are CATV companies and phone companies.
This is due to the two existing network infrastructures:
Telephone (POTS) networks (xDSL);
Cable TV networks (Cable Modems).
Since existing cable infrastructures are available, operators try to “milk”
their networks to provide data services such as high speed Internet
access.
This requires upgrading those infrastructures, to overcome the 4 kHz
limited unidirectional broadcast.
Services can be switched/broadcast, symmetrical/asymmetrical,
unidirectional/bidirectional, and can be provided with different
bandwidths.
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Services Supported by Access Networks
Service
Type
Downstream Bandwidth
Upstream Bandwidth
Telephony
Switched
4 kHz
4 kHz
ISDN
Switched
144 kbit/s
144 kbit/s
Video Broadcasting
Broadcast
6 MHz or 2 to 6 Mbit/s
0
Interactive Video
Switched
6 Mbit/s
Small
Internet Access
Switched
Some Mbit/s
Small (initially)
Video-Conference
Switched
6 Mbit/s
6 Mbit/s
Enterprise Services
Switched
1.5 Mbit/s to 10 Gbit/s
1.5 Mbit/s to 10 Gbit/s
Downstream: from the service provider to the user.
Upstream: from the user to the service provider.
Full Service: provision of all the services available through access networks.
Criteria for Classification of Services
Bandwidth requirements.
Symmetrical (bidirectional) or Asymmetrical (unidirectional).
Broadcast or Switched.
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Architectures for Access Networks
NIU
H = Hub
RN = Remote Node
NIU = Network Interface Unit
A NIU may be located in the
premises of one subscriber or
may serve several subscribers.
RN
H
The Hub is usually part of a larger network,
but may be considered as the source of
data for the NIUs and the destination for
Fe e de r Ne twork
data coming from the NIUs.
NIU
RN
RN
NIU
Dis tribution Network
Different combinations of services and network topologies are possible: a broadcast service
may be supported by a broadcast or switched network, as well as a switched service.
In a broadcast network, a RN broadcasts data it receives from the Hub to all its NIUS.
In a switched network, a RN processes the data coming from the Hub and, eventually,
sends different traffic flows to each of its NIUs.
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Classification of Access Networks
Distribution
Feeder
Network
Network
Shared BW
Dedicated BW
Broadcast
CATV (HFC), TPON
WPON
Switched
Telephony, DSL, WRPON
HFC = Hybrid Fiber Coaxial;
DSL : Digital Subscriber Loop;
PON : Passive Optical Network;
T = Telephony;
W : Wavelength;
WR = Wavelength Routed;
BW = BandWidth.
Broadcast networks may be cheaper than switched ones, are adequate for provision of broadcast services,
and have the advantage of identical NIUs for all the subscribers.
Switched Networks are more appropriate for switched services and provide more security.
Fault localization is generally easier in switched networks, because intelligence is in the network instead of
the NIUs, as in broadcast networks.
In switched networks the NIUs may be simpler than in broadcast networks.
A disadvantage of feeder networks with shared bandwidth is the need for each NIU to operate in the total
network bandwidth. Besides, with dedicated bandwidth it is possible to provide QoS guaranties to the NIUs.
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Local Telephone Network
CO = Central Office (USA) or Local Exchange (EU)
TPs = Twisted Pairs
(copper)
This is a switched network designed to provide a bandwidth of 4 kHz to each subscriber,
although modulation and signal processing techniques available nowadays enable the
use of much larger bandwidths.
The same infrastructure is used by the ISDN (BW of 144 kbit/s) and by DSL technology
(a few Mbit/s). With the latter, the possible data rate varies inversely with the distance to
the local exchange, and the 4 kHz filters of the original telephone line must be removed.
http://www.dslforum.org/
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CATV Network – HFC Architecture
A better designation to describe the HFC
architecture would be:
SMFCB = Subcarrier Modulated
Fibre Coaxial Bus
Distribution
Network
Feeder Network
HE
HE = Head End
Optical Fibre
RN
Coaxial Cable
Amplifier
Tap
The feeder network uses the signal of a laser with SCM (Sub Carrier Multiplexing).
The feeder network uses a bandwidth between 50 and 550 MHz. In this band, the coaxial
cable can carry about 70 TV signals in AM-VSB.
A spectral window between 5 and 40 MHz is also available, for return signals.
Optical Access Networks
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Alternative (broadband) Access Technologies
Satellite Systems
Systems for direct broadcasting by satellite (DBS) use geostationary satellites to broadcast
a few hundred TV channels, and may provide larger bandwidth than cable broadcasting
systems, but frequency reuse is limited due the large coverage area (footprint) of each
satellite, and it is not easy to provide support for upstream traffic.
Fixed Wireless Access (FWA)
Although with limited BW and reach, these systems may be deployed quickly and enable
new service providers to enter the market without a cable infrastructure.
The more relevant alternatives are MMDS (Multichannel Multipoint Distribution System) and
LMDS (Local Multipoint Distribution System). Both are line of sight systems.
MMDS provides more than 30 TV channels in the bandwidth from 2 to 3 GHz, with a reach of
15 to 55 km, depending on the transmitted power.
http://www.wcai.com/mmds.htm
LMDS operates in the 28 GHz band with a bandwidth of 1.3 GHz, and is more adequate for
short reach coverage (3 to 5 km, depending on rain) in dense urban areas.
http://www.lmdswireless.com/
Free Space Optics (Optical Wireless)
These systems employ lasers, and may provide a transmission capacity from about 100
Mbit/s up to 2.5 Gbit/s, with a reach from hundreds of m to a few km, in line of sight.
http://www.fsona.com/index.php
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Enhanced HFC (EHFC) Architecture
http://www.adventnetworks.com/products/Overview.pdf
Upstream
0 5
Downstream Video
40 52
Available for downstream traffic
550
862
1000
Frequency (MHz)
Improvements provided by the EHFC Architecture
The bandwidth may be increased up to 1 GHz. Systems using 862 MHz are already available.
In each channel, digital modulation techniques with high spectral efficiency may be used,
such as 256-QAM (7 (bit/s)/Hz).
The fibre may penetrate deeper into the network, thereby reducing the number of subscribers
served by each RN to about 50 (instead of 500, typical of the HFC architecture).
Multiple fibres and multiple wavelengths may be used (greater capacity).
Besides high power lasers combined with booster optical amplifiers in the wavelength of
1.55 µm, signals in the 1.3 µm window may be multiplexed in narrow band mode
(narrowcasting), for selected groups of users.
Functions of the NIUs
Each NIU may serve one or more subscribers, and its functions are: (1) to decompose the
signals into telephone and video signals, and (2) send them on twisted and coaxial pairs,
respectively, to each subscriber.
Optical Access Networks
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Fibre Access Networks: FTTCab, FTTC/B, FTTB/H
In contrast with the HFC architecture, in fibre access networks (Fiber In The Loop, FITL) data are
transmitted in digital format over optical fibre, in the distribution network, until fibre terminating nodes
designated by ONUs = Optical Network Units
The designations used to describe this architecture depend on the proximity between the ONUs and the
subscribers' premises:
FTTH (Fiber To The Home); ONUs perform the functions of NIUs;
FTTC (Fiber To The Curb) / FTTB (Fiber To The Building); the ONUs serve a small number of
subscribers (8 to 64); typically, the fibre is terminated at less than 100 m from the subscriber, and
there is an additional copper distribution network between ONUs and NIUs;
FTTCab (Fiber To The Cabinet); the fibre is terminated in a nearby cabinet, less than 1 km from the
subscriber, and a distribution network between ONUs and NIUs is also employed.
Central Office
Cabinet
RN
CO
Fiber
Curb
RN
CO
RN
by Henrique J. A. da Silva, 2005.03.09
FTTCab
NIU
FTTC/FTTB
ONU/NIU
FTTB/FTTH
Copper
ONU
Passive Optical Network (PON)
Optical Access Networks
NIU
ONU
Fiber
CO
Home
FTTP : Fiber To The Premises
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PON Architectures
PON = Passive Optical Network : network between the CO and the ONU.
The remote node (RN) is a simple passive device, such as an optical star coupler or a static wavelength
router, and sometimes may be installed at the central office.
The designation FTTC is commonly used to describe an architecture in which the signals are broadcast
from the CO to the ONUs, and the ONUs share the total bandwidth in TDM. This architecture is also
designated by BMFCB (Baseband Modulated Fiber Coaxial Bus) or SDV (Switched Digital Video).
In the context of FTTC architectures, the feeder network is the part of the network between the CO and the
RNs, and the distribution network is the part between the RNs and the ONUs.
Several types of architectures may be realized by employing different types of sources at the CO, combined
with different types of RNs.
Architecture
Shared
Fiber
Power
Splitting
Data Rate at
the ONU
Node
Synchronization
Shared
CO
AF (All Fiber)
No
None
1
No
No
TPON
Yes
1/N
N
Yes
Yes
WPON
Yes
1/N
1
Yes
No
WRPON
Yes
None
1
Yes
Yes
N : number of ONUs in the network.
Data Rate: 1 means that ONU operates at the rate of the traffic delivered to it, instead of the aggregate rate N.
Optical Access Networks
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AF (All Fiber) Architectures
Solution with point-to-point fibre
Solution with fibre in a ring
ONU
ONU
cable
ONU
ONU
CO
CO
ONU
ONU
ONU
ONU
The simplest PON architecture uses a pair of fibres between the CO and each ONU. Physically, the fibres
may be deployed in a ring configuration. Instead of a pair, a single fibre with bidirectional transmission
may be used, but the same wavelength cannot be used to simultaneously transmit in both directions, due
to reflections at the ends. The solution is to use Time Division Duplex (TDD) or different wavelengths,
(1.3 and 1.55 µm, for example), which may be called Wavelength Division Duplex (WDD)..
The cost of this solution grows linearly with the number of ONUs, and the operator must deploy and
maintain all the fibres.
Presently, this architecture is used in small scale, mainly to provide high speed services to enterprises
(NTT, in Japan, operates a system with data rates between 8 and 32 Mbit/s in each fibre).
In the SONET/SDH rings, a pair of fibres is shared by multiple ONUs (which are OADMs). These rings are
considered as an alternative access solution, but not as members of the PON family.
Optical Access Networks
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TPON (Telephony PON) Architecture
In spite of being a broadcast architecture, it may support switched
services by assigning specific time slots to each ONU, based on
its bandwidth needs.
In the distribution network, the upstream shares the fibre with the
downstream by using TDM, in a different wavelength, through a
coupler.
ONUs must be synchronized (ranging).
CO
Laser
1,55 µm
ONU
Rx: pin FET
Tx: LED or FP laser
Receiver
MUX/DX
1310/1550 nm
RN
Laser
1,3 µm
ONU
Receiver
Splitter/
Combiner
Receiver
The number of ONUs that can be
supported is limited by the splitting
losses at the RN.
ONU
MUX/DX
1310/1550 nm
Passive Star Coupler
Laser
1,3 µm
ONU
Receiver
ONUs must operate at the aggregate rate coming from the CO.
There is a trade off between transmitted power, receiver sensitivity,
data rate, number of ONUs, and maximum coverage distance.
Optical Access Networks
by Henrique J. A. da Silva, 2005.03.09
MUX/DX
1310/1550 nm
Laser
1,3 µm
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Commercial Opportunity for PONs
Presently, the higher data rates in the deployed access networks are offered by xDSL technologies and
Cable MODEMs, and they do not usually reach more than 1.5 Mbit/s.
Contrasting with this, the bandwidth grows dramatically in the transport network due to the adoption of
WDM technology, which is already penetrating into metropolitan networks (MANs).
Simultaneously, most organizational LANs have been upgraded from 10 do 100 Mbit/s, and it is
expectable that they will be upgraded to gigabit-Ethernet in the near future.
The result of these conditions is an ever larger gap between the capacity of metropolitan networks and
the needs of residential subscribers and SMEs (Small and Medium Enterprises).
PONs may fill this gap, satisfying the market needs in the interval of data rates from 1.5 Mbit/s to 155
Mbit/s, not yet supported by economical alternative technologies.
Optical Access Networks
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PON – Typical Architecture
A PON is formed by and Optical Line
Terminal (OLT), located at the CO,
and a set of ONUs located at or in the
neighbourwood of subscribers' premises.
Between these active elements an ODN
(Optical Distribution Network) is deployed,
employing optical fibres and passive
components (splitters/combiners and
couplers).
Presently, the ODN may operate at
155 Mbit/s, 622 Mbit/s, 1.25 Gbit/s, or
2.5 Gbit/s, by using one of the available
standards APON/BPON/EPON/GPON.
http://www.ponforum.org/
Downstream traffic is broadcast by the OLT to all ONUs. Each of these processes the traffic which is
destined to it, through an address contained in the header of the PDU (Protocol Data Unit).
Upstream traffic uses TDMA, under control of the OLT located at the CO, which assigns time slots to each
ONU for synchronized transmission of its data bursts.
The bandwidth assigned to each user may be static or dynamically variable, for support of voice, data and
video applications.
Optical Access Networks
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FSAN (Full Service Access Network) Standard
This standard was developed by an alliance of service providers and equipment builders,
and specifies a TPON architecture based on ATM (APON) with a downstream bandwidth
of up to 622 Mbit/s and an upstream bandwidth of up to 155 Mbit/s.
Maximum coverage distance is 20 km, with a total fibre attenuation between 10 and 30 dB.
Practical values of the link power budget allow power splitting by 16 or 32 at the RN.
For example, an APON operating at 622 Mbit/s with a 32-way splitter may provide a data
rate of 20 Mbit/s to each subscriber.
An APON may operate over a single fibre, by using different wavelengths in the upstream
(1.3 µm) and downstream (1.55 µm), or over a pair of fibres, by employing only 1.3 µm
transmitters.
http://www.fsanweb.org/
Optical Access Networks
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BPON (Broadband PON) and GPON (Gigabit-capable PON)
APONs (ATM-based PONs) were developed during the second half of the 1990 decade, through the
FSAN (Full Service Access Network) initiative, which resulted from the industrial alliance promoted by BT
in 1995 with the objective of developing standards to extend high speed services (such as data over IP,
video and Ethernet 10/100) over fibre to residential customers and SMEs.
The APON standard was primarily focused in applications for SMEs, and its initial version did not include
provision of video services over the PON.
The APON format developed by the FSAN alliance was used as the basis for an international standard
released by ITU-TS (Rec. G.893), designated by BPON (Broadband PON).
This standard supports more broadband services, including high-speed Ethernet and video distribution.
In 2001, the FSAN group initiated the development of the standard GPON (Gigabit-capable PON, Rec.
G.984, ITU-T, 2003), for full service support, including voice (TDM, over SONET/SDH), Ethernet (10/100
BaseT), ATM, leased lines, wireless extension, etc., by using a convergence protocol layer designated
GFP (Generic Framing Procedure).
The main characteristics of the GPON standard are:
Physical reach of at least 20 km, with support for logical reach up to 60 km;
Support of several data rate options, using the same protocol, including a symmetrical link at 622
Mbit/s or 1.25 Gbit/s, or 2.5 Gbit/s downstream with 1.25 Gbit/s upstream.
Management of end-to-end services with good capabilities of OAM&P (Operation, Administration,
Maintenance and Provisioning).
Security of downstream traffic at protocol level, due to the multicast nature of the PON.
Optical Access Networks
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GPON (Gigabit-capable PON) - Architecture
Optical Access Networks
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EPON (Ethernet PON)
When it was developed, the APON standard was not the more appropriate solution for local
access in large scale, because it did not include the capability to transport video.
On the other hand, evolution of LANs to gigabit Ethernet and 10-gigabit Ethernet is a strong
motivation to eliminate the need of conversion between IP and ATM protocols, in the
interconnection of LANs to WANs (Wide Area Networks).
In November 2000, a group of manufacturers of Ethernet equipment initiated the
development of an Ethernet PON (EPON) standard under an agreement with the IEEE,
with the creation of the the EFM (Ethernet in the First Mile) study group in 2001.
Although the EPON concept provides larger bandwidth, lower costs, and wider service
capabilities than the APON standard, the network architecture is similar and the EPON
standard complies to many specifications included in recommendations G.983/4 of ITU-T.
http://www.alloptic.com/products/whitepapers/031601.htm
The IEEE standard for Ethernet in the first mile is 802.3ah.
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WPON (WDM PON) Architecture
This architecture enables the operation of each ONU
at the data rate of the traffic delivered to it, instead
of the aggregate data rate transmitted by the CO.
However, it is still limited by the power splitting
at the star coupler (RN).
Tx at the CO:
Array of Lasers
or Tuneable Laser
CO
ONU
Filter λ
1
λ1 , λ2 , ... , λ N
Laser
1.3 µm
1.55 µm band
RN
ONU
Filter λ
2
WDM Laser
Receiver
Receiver
Splitter/
Combiner
Laser
1.3 µm
Receiver
ONUs share the upstream wavelength
of 1.3 µm, with TDM.
λ1 , λ2 , ... , λ N
1.55 µm band
ONU
Filter λ N
Receiver
Laser
1.3 µm
Optical Access Networks
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WRPON Architecture
The introduction of static wavelength routing solves the splitting
loss problem, maintaining all the advantages of the WPON.
λ1
ONU
Receiver
Besides, it allows provision of point-to-point dedicated
services.
Laser
1.3 µm
λ1 , λ2 , ... , λ N
1.3 or 1.55 µm band
CO
The AWG (Arrayed
Waveguide Grating)
operates as a static
wavelength router.
WDM Laser
Receiver
RN
AWG
by Henrique J. A. da Silva, 2005.03.09
ONU
Receiver
Laser
1.3 µm
Combiner
The first system demonstrated (PPL: Passive Photonics
Loop) used 16 channels in the 1.3 µm band, for downstream
traffic, and 16 additional channels in the 1.55 µm band,
for upstream transmission.
This architecture is not economical, because it needs two
expensive lasers for each ONU (including the one in the CO).
Optical Access Networks
λ2
λN
ONU
Receiver
Laser
1.3 µm
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RITENET WRPON Architecture
Each frame transmitted by
the CO includes a data field Transmitted frame
signal
unmodulated
and a field for return traffic, modulated
(downstream)
signal
during which the laser
remains switched ON.
λ1
Coupler
ONU
Receiver
MODULATOR
λ1 , λ2 , ... , λ N
1.55 µm band
CO
modulated signal (upstream)
RN
λ2
WDM Laser
Coupler
ONU
Receiver
AWG
WDM
Receiver
This architecture avoids having a laser
in each ONU, replacing it by an optical
modulator which reuses the signal
received from the CO.
If a single receiver is used at the CO,
the ONUs must use TDM to access it.
Optical Access Networks
by Henrique J. A. da Silva, 2005.03.09
MODULATOR
λN
Coupler
ONU
Receiver
MODULATOR
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LARNET WRPON Architecture
This is an economical alternative to RITENET that
uses a LED instead of the modulator, in the ONUs,
for upstream transmission.
The LED emits a broadband signal which spectrum
is split into different wavelengths by the AWG, with a
loss factor of at least 1/N (for N ONUs).
CO
λ1
ONU
Receiver
LED
1.3 µm
RN
λ2
LED
1.55 µm
ONU
Receiver
AWG
Receiver
1,3 µm
A LED may also be used at the CO, in which case
the signal emitted downstream is effectively broadcast
to all ONUs.
It is possible to have two transmitters at the CO: a 1.3 µm
LED, for example, broadcasting the signal to all ONUs,
and a 1.55 µm laser, transmitting only for selected ONUs.
This is an economical solution for broadcasting analog video
signals without the need of an overlay dedicated network.
Optical Access Networks
by Henrique J. A. da Silva, 2005.03.09
LED
1.3 µm
λN
ONU
Receiver
LED
1.3 µm
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Spectral Slicing
The broadband signal emitted by the LED is sent through the AWG.
Power
Only the fraction of the spectrum corresponding to the channel pass band of the AWG comes
out of each output.
λ1
Power
Power
Wavelength
Wavelength
LED
AWG
λ2
Wavelength
Power
data
λN
Wavelength
Optical Access Networks
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Scenario for PON Evolution
Grow As You Go Strategy
An operator may follow a progressive upgrading sequence, from a simple TPON
architecture towards one of the more complex WRPON architectures.
This evolution may be realized with minimum disruption of existing services and without
waste of installed equipment. The terminal equipment may be upgraded when new
services and/or additional capacity are required, without any change in the deployed
fibre network (grow as you go).
Possible Evolution Scenario for a PON
The operator installs a simple broadcast PON, which is a star network with bandwidth
shared by the ONUs.
If it becomes necessary to support more ONUs, the operator may upgrade the network
to a WPON, which is a broadcast network with dedicated bandwidth provided to each ONU.
The transmitters at the CO must be replaced by WDM transmitters, maintaining the same
ONUs.
If greater capacity per ONU becomes necessary, the operator may upgrade the network to
a WRPON, which is a switched network with dedicated bandwidth to each ONU.
The WRPON may also support broadcast services efficiently, by employing the spectral
slicing technique.
Optical Access Networks
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Future Developments
Capacity Increase (in both GPON and EPON)
Through combinations of TDM with WDM and, eventually,
CDMA; cheaper components are required (probably Coarse
WDM with polymer based AWGs and VCSEL arrays).
Through increase of data rate, with faster lasers and more
sensitive burst-mode receivers (with broadband SAGM APDs).
QoS in EPON
None of the currently existing EPON network access protocols
is capable of supporting variable burst and delay sensitive
multimedia streams and, in view of the increasing share of
multimedia in the overall IP traffic, this inability represents a
serious drawback and lack of future proof characteristics.
There is a need for introducing future-proof QoS policies along
with increased security measures to counteract any possible
network intrusion and data theft.
Optical Access Networks
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Bibliography
Books
Rajiv Ramaswami and Kumar N. Sivarajan, "Optical Networks – A
Practical Perspective" (second edition), Morgan Kaufmann
Publishers, Academic Press, 2002. (Chapter 11)
Denis J. G. Mestdagh, "Fundamentals of Multiaccess Optical Fiber
Networks", Artech House, 1995. (Part II)
Periodicals
You may start with the IEEE Communications Magazine
(the February 2005 issue includes two papers on EPON access
protocols).
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END
Thank You for your patience and attention.
Please send your queries and comments to:
[email protected]
Optical Access Networks
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